For single-color or multicolor printing of a recording medium (for example a single sheet or a belt-shaped recording medium) made of the most varied materials (for example paper or thin plastic or metal films) it is known to generate image-dependent charge images on an intermediate image carrier (for example a photoconductor), which charge images correspond to the images to be printed that are comprised of regions that are to be inked and regions that are not to be inked. The regions of the charge images that are to be inked are made visible via toner with a developer station. The toner image is subsequently transfer-printed onto the recording medium.
Liquid developer containing toner and carrier fluid can thereby be used to ink the charge images. Silicon oil is a possible carrier fluid, for example.
A method for electrophoretic liquid developing in digital printing systems is known from US 2007/212113 A1, for example. A carrier fluid containing silicon oil, with ink particles (toner) dispersed in it, is thereby used as a liquid developer. Further details in this regard can be learned from US 2007/212113 A1, which is incorporated into the disclosure of the present application. FIG. 1 shows the components of a printing system DS with a print group DW as it is known from US 2007/212113 A1, for example. Arranged along an intermediate image carrier 1 (a photoconductor in FIG. 1) are a regeneration exposure 2, a charging station 3, an element 4 for graphical exposure, a developer station 5 to develop the charge images into toner images, a transfer printing station 6 to transfer-print the toner images onto a recording medium 7, and an element 8 to clean the photoconductor drum 1. The transfer printing station 6 has an elastic transfer printing roller 60, a counter-pressure roller 61 and a cleaning unit 62. The developer station 5 rests, a reservoir 53 for the liquid developer, and a cleaning unit 55 cleaning the developer roller. The operation of the developer station is known from US 2007/212113 A1, for example, and reference is made to this.
In this printing system DS the transfer printing of the toner at the recording medium 7 is ensured via a carrier fluid layer between the transfer printing roller 60 and the recording medium 7. The toner crosses the carrier fluid layer from the transfer printing roller 60 to the recording medium 7, driven by electrical forces.
The complete wetting of the surface of the recording medium is required for the effectiveness of the transfer printing process since the toner otherwise cannot reach the surface of the recording medium and remains with the carrier fluid on the transfer printing roller. The toner cannot leave the carrier fluid layer since the electrical forces cannot overcome the surface forces. If a gap thus remains between the carrier fluid layer and the recording medium, the transfer printing at this location is blocked. This case can in particular occur given a fibrous and rough recording medium that has depressions that are markedly deeper than the carrier fluid layer, with the consequence that the depressions are no longer filled with carrier fluid. Furthermore, the take-up capability of the recording medium for the carrier fluid can be different. The recording medium takes up more or less carrier fluid depending on this. Therefore carrier fluid layers of different thickness are required in the transfer printing to the recording medium.
Furthermore, in electrophoretic printing systems with multiple print groups a problem exists with regard to the transfer printing to the recording medium between the first print group and subsequent print groups. In the first print group the recording medium is not wetted with carrier fluid before the transfer printing; in contrast to this, in the following print groups a carrier fluid layer that has already partially penetrated into the recording medium is already present on the recording medium. The take-up capability of the recording medium for additional carrier fluid in the subsequent print groups is thereby altered, with the result that the required carrier fluid supply is different in the subsequent print groups.
In U.S. Pat. No. 3,856,519 a method is described as to how the transfer printing to the recording medium can be improved given liquid developing. Here the problem is that the carrier fluid remains on the recording medium after the transfer printing of the toner images onto the recording medium, and therefore the recording medium is still wet after leaving the printer. The cause is that the carrier fluid has too high a boiling point. In order to avoid this problem, before the transfer printing a fluid that has a low boiling point is applied to the recording medium, with the consequence that this fluid vaporizes quickly after the transfer printing. It is therefore prevented that the carrier fluid arrives at the recording medium in transfer printing.